Interfacial redox couples involving minerals and dissolved species play important roles in controlling the fate and transport of nutrients and environmental pollutants in water. Despite much effort, modelling the extents and kinetics of these interfacial redox reactions has proven to be challenging due to the inability to quantify interfacial redox couples’ thermodynamic values. These values are difficult to obtain using electrochemical techniques because equilibration rates between redox-active minerals and electrodes are slow. Here, we demonstrate that this limitation can be overcome using mediated potentiometry, which employs soluble electron mediators that rapidly reach redox equilibria with both electrode surfaces and suspended mineral particles. We used mediated potentiometry to measure reduction potentials of goethite, hematite, and magnetite suspensions as a function of pH and aqueous Fe(II) concentration. The measured reduction potentials were then used to calculate the chemical potential energy of each iron oxide. The calculated chemical potential energies were in excellent agreement – within 1 kJ/mol – of recently measured values using calorimetric techniques. Since these measurements are relatively easy to make, we believe that this method could make thermodynamic measurements of redox-active minerals and interfacial redox couples simple and affordable, which would greatly enhance our ability to understand and predict extents and rates of interfacial redox reactions in the environment.

Thermodynamically characterizing interfacial iron redox couples using mediated potentiometry

251st National Meeting (2016)

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